A drop in U.S. carbon dioxide pollution in recent years stems from burning natural gas instead of coal. Or does it? Given that the U.S. bid to combat climate change through actions like the Clean Power Plan relies on more burning of gas than coal in power plants, that answer is both politically and scientifically important.

Compared with coal, burning natural gas results in roughly half the amount of CO2 per megawatt-hour of electricity. Yet even half the CO2, when spread over hundreds of power plants, is too much to achieve such goals as a CO2-emission reduction of 80 percent by 2050 or 100 percent by the end of this century, in order to avoid more than 2 degree Celsius of global warming, more acidic oceans, inexorable sea level rise and extreme weather, among other unpleasant impacts predicted by scientists. Under the terms of the Clean Power Plan, the most advanced natural gas burning power plants can still emit 771 pounds of CO2 per megawatt-hour of electricity produced. So is natural gas a bridge to a cleaner energy future or a slightly longer route to climate catastrophe?

To answer that question the past may provide a rough guide. At roughly the same time after the turn of the 21st century the U.S. underwent a recession, an energy transition to more natural gas and a move away from producing highly polluting products such as steel. So which of these factors deserves the most credit for the accompanying drop in the nation’s global warming pollution? It's an important puzzle to solve because recessions are not widely viewed as a policy option (advocates of “degrowth” notwithstanding) whereas the export of fracking—the process by which natural gas is extracted from shale rock—to other countries to help deliver a cleaner fossil fuel habit is.

Climate Code Red: The Case for Emergency Action is a 2008 book which presents scientific evidence that the global warming crisis is worse than official reports and national governments have so far indicated. The book argues that we are facing a "sustainability emergency" that requires a clear break from business-as-usual politics. The authors...